A variety of devices for separating fluid mixtures with hollow fiber membranes have been described. Typically, the separation process is carried out in a module fabricated from semi-permeable membranes. Such selective permeation results in the separation of the fluid mixture into retentate, i.e. slowly permeable components, and permeate portions, i.e. faster migrating components. The efficiency of the fluid separation process is determined by the properties of fluid mixture, relative permeabilities of various components of the fluid mixture, resulting from a gradient of driving forces, such as pressure, partial pressure, concentration and temperature, the membrane material and its structure. Preferably, the separation membrane is highly selective, i.e. the membrane has a high separation factor, high gas permeability, and is resistant to chemicals and temperature variations, and is mechanically strong. However, membranes with high selectivity are generally characterized by low permeability, while membranes with high permeability generally possess unacceptably low separation factors.
Attempts have been made to improve efficiency of the fluid separation process by developing new materials with better membrane transport properties, or by modifying the module design of the separation device. Generally, module designs consist of heat exchanger designs for controlling the contacting of internal flows. Conventional designs generally rely on co-current flow, counter-current flow, and cross flow contacting patterns for the low-pressure permeate and high-pressure retentate. See for e.g. U.S. Pat. Nos. 5,288,308; 5,176,725; 5,525,143; 5,013,331; 5,709,732; 4,964,886; 5,470,469; 5,137,631; 5,043,067 and 5,549,829.
The performance of these basic designs is improved by (1) permeate recycling, i.e. introducing a portion of one of the product streams back into the same module, or (2) module cascades, including the continuous membrane column, wherein a portion of one of the product streams back into a second module. Internal baffles or other design features permit cascading within a single fiber bundle. The internal staging design described by Kimura et al. (S. Kimura, et al., Radiochem. Radioanal. Lett., 1973, 13:349-354) consists of membranes made from two different materials, permitting contacting the feed with both membranes simultaneously, wherein each membrane possesses different permselectivity. Thus this design has limited utility, because materials with the requisite transport properties are available only for a limited number of separations. Additionally, although in comparison to external staging, internally staged designs require less plumbing and fewer module housings, the cost of manufacture is higher due to increased complexity.
Therefore, current membrane devices containing membrane modifications, and internally staged designs have several disadvantages and thus are not commercially viable for meeting current uses. Thus, there is a need for improved and cost-effective devices comprising fiber membranes that are capable of operating at acceptable levels of separation productivity.
The current method provides a cost-effective gas separation membrane device with a significant improvement in selectivity with a commercially acceptable loss of productivity.